1. Field of the Invention
The present invention relates to linkers on biochips, and more particularly, to a linker of bridging an electrode and a capture probe on a biochip.
2. Description of Related Art
A biochip is a chip designed to detect or quantify a target analyte such as protein, DNA, cell, glucose, cardiomyopathic biomarkers (e.g., S100 and C-Reactive protein (CRP), Troponin I, CKMB and the like), cancer biomarkers (e.g., cancer antigen 125 (CA125), carcinoembryonic antigen (CEA), prostate-specific antigen (PSA) and the like), bacterium biomarkers (e.g., E. Coli, staphylococcus and the like), and virus (e.g., H1N1 and the like. Many biochips are affinity-based that use an immobilized capture probe to bind the target analyte and then use a transducer to detect a change at a localized surface. The capture probe is a bio-recognition element that could bind or interact with the target analyte. For example, the capture probe may be an antibody when the target analyte is an antigen. The change could be measured in a variety of ways. Among these selections, an impedance biochip that measures the electrical impedance of an interface between an electrode and a fluid sample has the advantages of label free, low cost, low power consumption and ease of miniaturization. Due to these benefits, the impedance biochip is more suitable for application where small size and cost issues are crucial, such as point-of-care diagnostics.
The impedance biochip can be divided into two groups: non-faradaic and faradaic. A non-faradaic biochip, also called a capacitance biochip, detects the concentration of target analyte (e.g., biomarker) by measuring the non-faradaic transient current or capacitance change of the electrode-solution interface. Capacitance difference corresponding to various analytes comes from the changes in the dielectric constant, charge distribution, dimension and shape when the biomarker-capture probe interaction occurs on the electrode-solution interface. The capacitance biochip has a drawback that forming a defect-free bio-recognition layer is more crucial than that in the faradaic biochip, because if the defect-free boo-recognition layer is not well insulated, ions can move through a defect area, and cause short circuiting leading to a decrease or absence of the signal.
In contrast, the faradaic biochip measures the faradaic current changes caused by the oxidation/reduction (redox) reaction of redox-active species. Thus, faradaic biochip requires the addition of a redox-active species. Steric hindrance resulted from binding between the immobilized capture probe and the target analyte will form a barrier to stop the diffusion of redox-active species from the solution to the electrode, therefore, fewer faradaic current will be detected when there are more target analyte in the fluid sample. By further calculating a current-voltage ration that gives the impedance, one can easily correlate the concentration of the target analyte with the calculated impedance.
A linker is the special molecular utilized to immobilize the capture probe on a biochip. It is critical that the capture probe be attached to the biochip surface in a way that maintains probe specificity and activity while inhibiting nonspecific binding. The most common types of linkers are based on thiols bound to gold surfaces and silanes to oxide surfaces. For biochips that adopt surface plasmon resonance or quartz crystal microbalance to measure the target analyte correlated change, a tightly-packed self-assembled monolayers (SAMs) is preferred in order to block access of most solution species to the electrode in the biochip. Linker with long chain thiol, such as an alkanethiol like 12-mercaptododecanoic acid (HS(CH2)11COOH, “12-MCA”), is more desirable. However, in the case of faradic impedance biochip where the electrode surface of the impedance biochip needs to be accessible to the redox species, these conventional adopted linkers are not suitable anymore.
In this regard, there is still a need from the industries to develop a linker for joining the capture probe to an electrode of the impedance biochip, which has higher conductivity and lower block effect for the redox-active species.